Index: binaries/data/mods/public/gui/session/trade/TradeButtonManager.js
===================================================================
--- binaries/data/mods/public/gui/session/trade/TradeButtonManager.js
+++ binaries/data/mods/public/gui/session/trade/TradeButtonManager.js
@@ -8,9 +8,6 @@
 		if (!TradeButtonManager.IsAvailable())
 			throw "TradeButtonManager instantiated with no tradeable resources or too few buttons!";
 
-		// Assume that the simulation state will always follow the GUI
-		this.tradingGoods = Engine.GuiInterfaceCall("GetTradingGoods", g_ViewedPlayer);
-
 		let resourceCodes = g_ResourceData.GetTradableCodes();
 		this.selectedResource = resourceCodes[0];
 		this.buttons = resourceCodes.map((resCode, i) => new TradeButton(this, resCode, i));
@@ -23,6 +20,9 @@
 
 	update()
 	{
+		// Update the values, since observers don't get the changes via the gui.
+		this.tradingGoods = Engine.GuiInterfaceCall("GetTradingGoods", g_ViewedPlayer);
+
 		this.tradeHelp.tooltip = colorizeHotkey(translate(this.TradeSwapTooltip), "session.fulltradeswap");
 
 		let enabled = controlsPlayer(g_ViewedPlayer);